JP3944578B2 - Strain and AE measuring device using optical fiber sensor - Google Patents
Strain and AE measuring device using optical fiber sensor Download PDFInfo
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- JP3944578B2 JP3944578B2 JP2003172321A JP2003172321A JP3944578B2 JP 3944578 B2 JP3944578 B2 JP 3944578B2 JP 2003172321 A JP2003172321 A JP 2003172321A JP 2003172321 A JP2003172321 A JP 2003172321A JP 3944578 B2 JP3944578 B2 JP 3944578B2
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- 239000013307 optical fiber Substances 0.000 title claims description 18
- 238000005259 measurement Methods 0.000 claims description 31
- 238000001514 detection method Methods 0.000 claims description 27
- 238000002834 transmittance Methods 0.000 claims description 23
- 238000010586 diagram Methods 0.000 description 18
- 230000003287 optical effect Effects 0.000 description 14
- 230000006378 damage Effects 0.000 description 3
- 239000000835 fiber Substances 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 2
- 238000000605 extraction Methods 0.000 description 2
- 238000005070 sampling Methods 0.000 description 2
- 238000001228 spectrum Methods 0.000 description 2
- 230000005540 biological transmission Effects 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 238000007689 inspection Methods 0.000 description 1
- 238000000034 method Methods 0.000 description 1
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
- G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
- G01L1/246—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using integrated gratings, e.g. Bragg gratings
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/165—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by means of a grating deformed by the object
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/18—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge using photoelastic elements
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Length Measuring Devices By Optical Means (AREA)
- Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
- Length Measuring Devices Characterised By Use Of Acoustic Means (AREA)
- Optical Transform (AREA)
Description
【0001】
【発明の属する技術分野】
本発明はファイバ・ブラッグ・グレーティング(以下「FBG」という。)センサを用いてひずみ変化を検出するとともに、材料・構造体の微視損傷発生にともなう弾性波放出(アコースティック・エミッション。以下、「AE」という。)を検出するものである。
【0002】
そしてこの発明は、圧電素子を用いて弾性波を発生させ構造体の健全性評価を行う際に、さらには衝撃負荷による高速なひずみ変化を検出する際に適用することができる。
【0003】
即ち、本発明は、材料や構造体の負荷、および損傷状態を調べるためのひずみと微視破壊発生にともなうAEを一つのFBGセンサで同時に計測する際に適用することができる。本発明は、自動車、航空機、橋梁、建築物などの健全性評価への利用が期待されるものである。
【0004】
【従来の技術】
従来、AEの検出には圧電素子を用いて、衝撃負荷の検出にはひずみゲージを用いて計測される技術が用いられている。
【0005】
又、FBGセンサからの反射波をFBGセンサのブラッグ波長とほぼ等しいブラッグ波長を有するFBGに通して、その透過光からAEを検出する手法が米国で提案されている(非特許文献1参照)。
【0006】
さらに、FBGセンサのブラッグ波長変化に関しては従来、FBGセンサからの反射波波長を光スペクトルアナライザーにより計測してひずみを測定している。
【0007】
【非特許文献1】
I. Perez, H.-L. Cui and E. Udd, 2001 SPIE, Vol. 4328, p.209-215
【0008】
【発明が解決しようとする課題】
しかしながら、従来のAEの検出に圧電素子を用いる技術は、計測パラメータが直接電気信号に変換されて計測するため、電磁波障害の影響を受ける欠点があった。
【0009】
又、FBGセンサは計測パラメータを光信号に変換するため電磁波障害を受けないが、検出された波形はかならずしもAEの原波形を再現することができず、波形にひずみが現れる場合がある。
【0010】
さらに、FBGセンサのブラッグ波長変化を光スペクトルアナライザーを用いて計測する技術は、光スペクトルアナライザーのサンプリング速度は通常、毎秒1サンプリング程度であるこのため衝撃荷重による高速なひずみ変化やAEのような数百kHzの周波数特性を持つ微小なひずみ変化を追随して検出することはできない。このため高速のひずみ変化を追随して検出することはできないという問題がある。
【0011】
本発明は、このような従来の問題点を解決することを目的とするものであり、次のような特徴を備えた光ファイバひずみセンサを実現することを課題とする。
(1)AEや衝撃負荷による高速なひずみ変化をFBGセンサで検出する際に正確にひずみ変化を検出することができる
(2)フィルタの反射特性を変えることにより、FBGセンサ一本で微小なひずみ変化であるAEから大きなひずみ変化が生じる衝撃負荷までの幅広いひずみ変化を検出することができる。
(3)FBGセンサは計測パラメータを光信号に変換するため電磁波障害を受けない。
(4)一個のFBGセンサでひずみとAEの両方を計測するもので、材料・構造物健全性評価の際に重要なひずみ、AE計測においてセンサ数を少なくする。
【0012】
【課題を解決するための手段】
本発明は上記課題を解決するために、FBGを書き込んだ光ファイバから成り被検体に取り付けられるFBGセンサと、該FBGセンサに広帯域波長光を入射するための広帯域光源と、上記FBGセンサから伝送される反射光を分岐するカップラーと、該カップラーで分岐された反射光をそれぞれ反射又は透過させるひずみ計測用のフィルタ及びAE検出用のフィルタと、を備えて成る光ファイバセンサを用いたひずみとAEの計測装置であって、上記ひずみ計測用のフィルタ及びAE検出用のフィルタは、二種類の波長のそれぞれに対応して互いに透過率が異なり、上記ひずみ計測用のフィルタ及びAE検出用のフィルタの透過光又は反射光は、ブラッグ波長の変化により強度が変化し、これらを光電変換器で電気信号に変換してひずみ変化とAEを同時に検出することを特徴とする光ファイバひずみとAEの計測装置を提供する。
【0013】
上記光電変換器で電気信号に変換して得られたひずみ変化に係る情報に基づいて、上記AE検出用のAE検出用のフィルタの透過率が変化する波長帯を制御し、上記AE検出用のフィルタの透過光強度、反射光強度、又は透過光強度と反射光強度の差からAEを計測することができる。
【0014】
【発明の実施の形態】
本発明に係る光ファイバセンサを用いたひずみとAE計測装置の実施の形態を実施例に基づいて図面を参照して以下説明する。
【0015】
(FBGの動作原理)
本発明の説明に入る前に、本発明の基本となっているFBGの原理を図1において説明する。広帯域光源からの光を光サーキュレータの端子▲1▼から、端子▲2▼につながれたFBGセンサ(FBGを書き込んだ光ファイバ)に入射する。
【0016】
FBGセンサからは、屈折率nと屈折率変化の間隔Λの積の二倍で与えられるλBを中心波長(本明細書ではこれを「ブラッグ波長」と呼ぶ。)とする狭帯域の光成分が反射され、それ以外の光成分はFBGセンサを透過する。光サーキュレータは図に示すように端子▲2▼につながれたFBGセンサからの反射光を端子▲3▼へ送る。
【0017】
図2は、ブラッグ波長とFBGセンサの受けたひずみとの関係を示す図である。FBGセンサがひずみを受けたとき、屈折率変化を設けた間隔と屈折率が変化する。今、FBGセンサがファイバ軸方向にεのひずみを受けたとき、ブラッグ波長λBの変化ΔλBは、一定温度条件下で、次の数1で与えられることが知られている。
【0018】
【数1】
【0019】
ここでpeは光弾性定数(=0.213)であり、εはFBGが受ける光ファイバ軸方向ひずみである。従って、FBGセンサが引っ張りひずみを受けたとき、ブラッグ波長は長波長側へ、圧縮ひずみをうけたときは短波長側へ移動する。例えば、ブラッグ波長1550nmのFBGセンサが1×10-6のひずみ変化を受けたとき、1.2pmだけブラッグ波長が変化(シフト)する。要するに、FBGセンサからの反射波の中心波長はFBGが受けるひずみ変化に比例して変動する。
【0020】
(本発明の特徴)
【0021】
本発明の特徴を以下説明する。本発明では、高速にブラッグ波長の変化を計測するために、波長に伴い透過率の異なるフィルタにFBGセンサからの反射光を通し、ブラッグ波長の変化を光強度の変化に変換する。
【0022】
そして、FBGセンサからの反射光を光サーキュレータを介して1×2カップラーにより、二つのファイバに伝送分岐し、それぞれ二種類の波長にともない透過率の異なるひずみ計測用フィルタとAE検出用のフィルタに通す。両方のフィルタの透過光及び反射光はブラッグ波長の変化により強度が変化する。これらを光電変換器で検出することにより、ひずみ変化とAEを同時に検出することができる。
【0023】
ひずみ計測用フィルタはAE検出用フィルタと比較すると広い波長範囲で透過率が変化する特性を有するものとする。ひずみなしでのブラッグ波長が1550nmのFBGセンサでは1×10-6ひずみ当たり1.2pmの波長シフトが生じることから、被検査対象物が最大±1%のひずみを受けることが想定される場合、ひずみにより±12nmのブラッグ波長変化が生じることになる。そのため1538〜1562nmの波長範囲で透過率が変動するフィルタによりひずみ変化が測定できる。
【0024】
FBGをAE検出用フィルタとして用いて、この計測システムからAEを検出する構成は、本発明者が、すでに特願2002−340197において提案している。AE検出のためにはFBGセンサからの反射波長帯域がAE検出用フィルタの透過率変化波長帯域中にあることが必要である。
【0025】
AE検出用フィルタの透過率変化は0.4 nm以内程度の非常に狭い波長域に限られることから、大きなひずみ変化が生じた場合においてはセンサからの反射光のブラッグ波長が大きく変動して、AE検出用フィルタの透過率変化波長範囲から外れることがある。このためAE検出用フィルタはその透過率変化波長帯が被検査物が受けるひずみに応じて変化するチューナブルフィルタであることが好ましい。
【0026】
ひずみ計測用フィルタの透過光、および反射光を光電変換器により電気信号に変換して、ひずみを計測する。このひずみ情報に基づいて、AE検出用のチューナブルフィルタの動作波長域(透過率が変化する波長帯)を制御する。 そしてチューナブルフィルタの透過光、または反射光強度、または透過光強度と反射光強度の差からAEを計測することができる。
【0027】
このシステムによりひずみ計測用フィルタを通した信号からひずみが計測され、AE検出用フィルタを通した信号からAEが計測される。以下、さらに図面を利用して本発明の実施例1〜3を説明をする。
【0028】
(実施例1)
図3は、本発明に係る光ファイバセンサを用いたひずみとAEの計測装置を説明する図である。この図3において、広帯域光源からの光を光サーキュレータを介して、FBGセンサに入射する。FBGセンサは被測定物に固定されている。
【0029】
FBGセンサからの反射光は光サーキュレータを介して、1×2カップラーに通される。1×2カップラーはFBGセンサからの反射光を2本の光ファイバに分岐する。一本の光ファイバはひずみ計測用のフィルタへ、もう一本はAE検出用のチューナブルフィルタへつながっている。
【0030】
それぞれのフィルタの透過光、および反射光は光電変換器につながれ、それぞれの光強度は電気信号に変換される。フィルタの反射光はフィルタの前段に光サーキュレータをつけることにより取り出すことが出来る。ひずみ計測用フィルタの透過光強度と反射光強度からひずみを計測することができる。
【0031】
光電変換器Sstはチューナブルフィルタ制御部に接続されている。チューナブルフィルタ制御部ではひずみ変化により移動したブラッグ波長を評価し、チューナブルフィルタの動作波長域(透過率が変化する波長域)制御のための信号をチューナブルフィルタへ送る。
【0032】
図4は、ひずみ計測用フィルタを用いたひずみ計測原理を示す図である、図4において、FBGセンサがひずみを受けるとブラッグ波長が変化する。透過率が波長にともない変化するフィルタにFBGセンサからの反射光を通して得られる透過光、および反射光強度はブラッグ波長の位置により変化する。
【0033】
例えば、図4の上の図に示すように、長波長になるに従い透過率が減少するフィルタにFBGセンサからの反射光を通した場合、FBGセンサが圧縮ひずみを受けた場合(ブラッグ波長が短波長側へシフト)、フィルタの透過光強度は増加する。
【0034】
また、図4の下の図に示すように、引張ひずみを受けた場合(ブラッグ波長が長波長側へシフト)、フィルタの透過光強度は低下することになる。この透過光強度変化を光電変換器により電気信号に変換することにより、電気信号強度としてブラッグ波長変化を評価することができる。
【0035】
また反射光強度に関しては反射率=1−透過率の関係から同じ原理によりブラッグ波長変化にともない信号強度が変化する。つまりフィルタの透過光強度と反射光強度はひずみにより変化することになる。しかし光電変換器が受光する強度は光ファイバコネクタの接続のたびに変化する。これはコネクタ接続部のアライメントのずれから引き起こされる。このため透過光、または反射光強度単独ではひずみを定量評価することはできない。透過光強度と反射光強度の差を両強度の和で割った値からひずみを定量評価することができる。
【0036】
図5の上の図は、FBGセンサによるAEの検出原理を説明する図である。AEによるひずみ変化は微小なのでFBGセンサによりAEを検出するためにはひずみ計測と比較して透過率が変化している波長域が狭いフィルタが必要である。FBGセンサからの反射光のブラッグ波長はAEにより微小ではあるが変化を受ける。
【0037】
このブラッグ波長変化を狭帯域の透過率変化をもつフィルタに通すことにより強度変化に変換させる。例えば図5に示すようにAEがない場合にFBGセンサからはブラッグ波長λsの反射光が戻ってきて、AE検出用フィルタの透過率変化の中心波長をλFとする。FBGセンサがAEによるひずみ変化を受けることによりブラッグ波長は変化する。ブラッグ波長は圧縮、および引張ひずみではそれぞれλs ’、 およびλs ’’に変化する。
【0038】
フィルタの透過光強度はAEによるひずみ変化により斜線で表される面積に比例して変化する。そのためAEによるひずみ変化によりフィルタの透過光強度を電気信号に変換する光電変換器の出力は図5の下図のようになる。反射光強度に関しても、反射率=1−透過率の関係から同様にAEを検出したとき反射光強度は変化する。また透過光と反射光信号強度の両者の差も同じ原理でAEにより変化する。
【0039】
AEを検出するためのフィルタとして誘電体多層膜フィルタ、FBGがある。またこの図の例ではAE検出用フィルタにバンドパスフィルタを挙げたが、ローパス、ハイパスフィルタを用いても良い。
【0040】
図6は、ひずみ変化にともなうチューナブルフィルタ動作波長帯の移動を示す図である。AEを検出するためのフィルタは透過率が変化している波長域が0.4nm程度である。
【0041】
このため大きなひずみ変化を受け、ブラッグ波長が数nm移動したときはAE検出用のフィルタの透過率が変化している波長域からFBGセンサの反射光の波長域は外れてしまうことになる。このためAE計測のためにはひずみ計測用のフィルタにより計測されたひずみ変化に応じてAE計測用のフィルタの動作波長域を変化させる必要がある。
【0042】
チューナブルフィルタは外部からの制御信号により、動作波長域を変化させることができる。AE計測用のフィルタとしてチューナブルフィルタとすることにより、FBGセンサが受けるひずみに応じてAE計測用のフィルタの動作波長域を制御する。このときチューナブルフィルタの動作波長域の制御にひずみ計測用のフィルタの透過光強度と反射光強度の差を両強度の和で割った値から評価されるひずみ情報を用いる。
【0043】
(実施例2)
図7は、本発明の実施例2を示し、複数のFBGセンサによる同時多点ひずみ、AE計測可能とする構成である。ブラッグ波長の異なるFBGセンサを直列に配列することにより多点で同時にひずみとAEを計測する装置を示す。FBGセンサ列からの反射光は光分波器によりそれぞれのFBGセンサからの信号に分離されて出力される。なお、図7においては図面の簡略化のためにフィルタ前段の光サーキュレータとフィルタの反射光の取り出しは図示しない。
【0044】
図8は、本発明の実施例3を示し、複数のFBGセンサによる特定箇所のひずみ、AE計測可能とする構成を示す。ブラッグ波長の異なるFBGセンサを直列に配列し、特定のFBGセンサが貼り付けられている箇所のひずみとAEを計測する装置を示す。
【0045】
光サーキュレータからのFBGセンサ列からの反射光をチューナブルフィルタを通して、所望するFBGセンサからの反射光成分のみを取り出す。また、ひずみ計測用のフィルタの動作波長帯もそれに連動させて変化させる。なお、図8においては、図面の簡略化のためにフィルタ前段の光サーキュレータとフィルタの反射光の取り出しは削除して表現している。
【0046】
以上、実施例により本発明に係る光ファイバセンサを用いたひずみとAE計測装置の実施の形態を実施例に基づいて説明したが、このような実施例に限定されることなく、特許請求の範囲記載の技術的事項の範囲内でいろいろ実施例があることは言うまでもない。
【0047】
【発明の効果】
以上の構成から成る光ファイバセンサを用いたひずみとAE計測装置によると、FBGセンサを用いてひずみとAEの両方を同時に一個のセンサで計測することができる。
【図面の簡単な説明】
【図1】 FBGの原理図を説明する図である。
【図2】ブラッグ波長とひずみとの関係を説明する図である。
【図3】本発明の実施例1を説明する図である。
【図4】実施例1の作用を説明する図である。
【図5】実施例1の作用を説明する図である。
【図6】実施例1の作用を説明する図である。
【図7】本発明の実施例2を説明する図である。
【図8】本発明の実施例3を説明する図である。[0001]
BACKGROUND OF THE INVENTION
In the present invention, a fiber Bragg grating (hereinafter referred to as “FBG”) sensor is used to detect a change in strain, and acoustic wave emission (acoustic emission, hereinafter referred to as “AE” associated with occurrence of microscopic damage of a material / structure). ").) Is detected.
[0002]
The present invention can be applied when an acoustic wave is generated using a piezoelectric element to evaluate the soundness of a structure, and further when a high-speed strain change due to an impact load is detected.
[0003]
That is, the present invention can be applied to simultaneously measuring strain for examining the load of a material or a structure and a damage state and AE associated with occurrence of microscopic destruction with one FBG sensor. The present invention is expected to be used for soundness evaluation of automobiles, aircraft, bridges, buildings, and the like.
[0004]
[Prior art]
Conventionally, a technology has been used in which AE is detected using a piezoelectric element, and impact load is detected using a strain gauge.
[0005]
Further, a method has been proposed in the United States in which a reflected wave from an FBG sensor is passed through an FBG having a Bragg wavelength substantially equal to the Bragg wavelength of the FBG sensor and AE is detected from the transmitted light (see Non-Patent Document 1).
[0006]
Furthermore, regarding the Bragg wavelength change of the FBG sensor, conventionally, the reflected wave wavelength from the FBG sensor is measured by an optical spectrum analyzer to measure the distortion.
[0007]
[Non-Patent Document 1]
I. Perez, H.-L.Cui and E. Udd, 2001 SPIE, Vol. 4328, p.209-215
[0008]
[Problems to be solved by the invention]
However, the conventional technique using a piezoelectric element for detecting AE has a drawback that it is affected by electromagnetic interference because measurement parameters are directly converted into electric signals for measurement.
[0009]
In addition, the FBG sensor does not suffer from electromagnetic interference because it converts the measurement parameter into an optical signal, but the detected waveform cannot always reproduce the original waveform of AE, and the waveform may be distorted.
[0010]
In addition, the technology that measures the Bragg wavelength change of an FBG sensor using an optical spectrum analyzer usually has a sampling rate of about 1 sampling per second. It is impossible to detect and detect minute changes in strain with a frequency characteristic of 100 kHz. For this reason, there is a problem that a high-speed strain change cannot be detected following up.
[0011]
An object of the present invention is to solve such a conventional problem, and an object of the present invention is to realize an optical fiber strain sensor having the following characteristics.
(1) High-speed strain change due to AE or impact load can be detected accurately when the FBG sensor is detected. (2) By changing the reflection characteristics of the filter, a small strain can be obtained with a single FBG sensor. It is possible to detect a wide range of strain changes from the AE that is a change to the impact load where a large strain change occurs.
(3) Since the FBG sensor converts measurement parameters into optical signals, it is not subject to electromagnetic interference.
(4) A single FBG sensor measures both strain and AE, and reduces the number of sensors in strain and AE measurement, which are important for evaluating the soundness of materials and structures.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention provides an FBG sensor made of an optical fiber in which FBG is written and attached to a subject, a broadband light source for making broadband wavelength light incident on the FBG sensor, and transmitted from the FBG sensor. A strain sensor using an optical fiber sensor, and a strain measurement filter and an AE detection filter for reflecting or transmitting the reflected light branched by the coupler, respectively. The strain measurement filter and the AE detection filter have different transmittances corresponding to each of the two types of wavelengths, and the transmission of the strain measurement filter and the AE detection filter is different. The intensity of light or reflected light changes due to the change in Bragg wavelength, and these are converted into electrical signals by a photoelectric converter to detect strain change and AE simultaneously. We provide a measuring device for characteristic optical fiber strain and AE.
[0013]
Based on the information on the distortion change obtained by converting the photoelectric signal into the electrical signal by the photoelectric converter, the wavelength band in which the transmittance of the AE detection filter for the AE detection changes is controlled, and the AE detection The AE can be measured from the transmitted light intensity of the filter, the reflected light intensity, or the difference between the transmitted light intensity and the reflected light intensity.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of a strain and AE measuring apparatus using an optical fiber sensor according to the present invention will be described below with reference to the drawings based on examples.
[0015]
(FBG operation principle)
Prior to the description of the present invention, the principle of FBG, which is the basis of the present invention, will be described with reference to FIG. Light from the broadband light source enters the optical circulator terminal (1) through the FBG sensor (optical fiber into which the FBG is written) connected to terminal (2).
[0016]
From the FBG sensor, a narrow-band light component having a central wavelength (referred to as “Bragg wavelength” in this specification) λ B given by twice the product of the refractive index n and the refractive index change interval Λ. Are reflected, and other light components pass through the FBG sensor. As shown in the figure, the optical circulator sends the reflected light from the FBG sensor connected to the terminal (2) to the terminal (3).
[0017]
FIG. 2 is a diagram showing the relationship between the Bragg wavelength and the strain received by the FBG sensor. When the FBG sensor is distorted, the distance between the refractive index changes and the refractive index change. Now, when the FBG sensor is subjected to a strain ε in the fiber axis direction, the change [Delta] [lambda] B of the Bragg wavelength lambda B is a constant temperature conditions, it is known that given by the following equation 1.
[0018]
[Expression 1]
[0019]
Here, p e is a photoelastic constant (= 0.213), and ε is an optical fiber axial strain that the FBG receives. Therefore, the Bragg wavelength moves to the long wavelength side when the FBG sensor is subjected to tensile strain, and to the short wavelength side when it is subjected to compressive strain. For example, when an FBG sensor with a Bragg wavelength of 1550 nm undergoes a strain change of 1 × 10 −6 , the Bragg wavelength changes (shifts) by 1.2 pm. In short, the center wavelength of the reflected wave from the FBG sensor fluctuates in proportion to the strain change experienced by the FBG.
[0020]
(Features of the present invention)
[0021]
The features of the present invention will be described below. In the present invention, in order to measure the change in the Bragg wavelength at high speed, the reflected light from the FBG sensor is passed through filters having different transmittances according to the wavelength, and the change in the Bragg wavelength is converted into a change in light intensity.
[0022]
Then, the reflected light from the FBG sensor is split into two fibers via a 1x2 coupler via an optical circulator, and each of them is used as a strain measurement filter and an AE detection filter with different transmittances according to two wavelengths. Pass through. The transmitted light and reflected light of both filters change in intensity due to the change in Bragg wavelength. By detecting these with a photoelectric converter, strain change and AE can be detected simultaneously.
[0023]
The strain measurement filter is assumed to have a characteristic that the transmittance changes in a wide wavelength range as compared with the AE detection filter. An FBG sensor with a Bragg wavelength of 1550 nm without strain produces a wavelength shift of 1.2 pm per 1 × 10 −6 strain, so if the object to be inspected is expected to be strained up to ± 1%, As a result, a Bragg wavelength change of ± 12 nm occurs. Therefore, the strain change can be measured by a filter whose transmittance varies in the wavelength range of 1538 to 1562 nm.
[0024]
The present inventor has already proposed in Japanese Patent Application No. 2002-340197 a configuration for detecting AE from this measurement system using FBG as an AE detection filter. For AE detection, it is necessary that the reflection wavelength band from the FBG sensor is in the transmittance change wavelength band of the AE detection filter.
[0025]
Since the transmittance change of the filter for AE detection is limited to a very narrow wavelength range of about 0.4 nm or less, the Bragg wavelength of the reflected light from the sensor fluctuates greatly when a large strain change occurs, and AE detection May be out of the transmittance change wavelength range of the filter. For this reason, the AE detection filter is preferably a tunable filter whose transmittance change wavelength band changes in accordance with the strain that the inspection object receives.
[0026]
Strain is measured by converting the transmitted light and reflected light of the strain measurement filter into electrical signals by a photoelectric converter. Based on this distortion information, the operating wavelength range (wavelength band in which the transmittance changes) of the AE detection tunable filter is controlled. The AE can be measured from the transmitted light or reflected light intensity of the tunable filter, or the difference between the transmitted light intensity and the reflected light intensity.
[0027]
With this system, strain is measured from the signal passed through the strain measurement filter, and AE is measured from the signal passed through the AE detection filter. Examples 1 to 3 of the present invention will be described below with reference to the drawings.
[0028]
Example 1
FIG. 3 is a diagram for explaining a strain and AE measuring apparatus using the optical fiber sensor according to the present invention. In FIG. 3, light from a broadband light source enters an FBG sensor via an optical circulator. The FBG sensor is fixed to the object to be measured.
[0029]
Reflected light from the FBG sensor is passed through a 1 × 2 coupler via an optical circulator. The 1 × 2 coupler branches the reflected light from the FBG sensor into two optical fibers. One optical fiber is connected to a strain measurement filter, and the other is connected to a tunable filter for AE detection.
[0030]
The transmitted light and reflected light of each filter are connected to a photoelectric converter, and the intensity of each light is converted into an electrical signal. The reflected light of the filter can be taken out by attaching an optical circulator in front of the filter. Strain can be measured from the transmitted light intensity and reflected light intensity of the strain measurement filter.
[0031]
The photoelectric converter Sst is connected to the tunable filter control unit. The tunable filter control unit evaluates the Bragg wavelength moved due to the strain change, and sends a signal for controlling the operating wavelength range (wavelength range in which the transmittance changes) of the tunable filter to the tunable filter.
[0032]
FIG. 4 is a diagram showing the principle of strain measurement using a strain measurement filter. In FIG. 4, when the FBG sensor is distorted, the Bragg wavelength changes. The transmitted light obtained through the reflected light from the FBG sensor through the filter whose transmittance varies with wavelength, and the reflected light intensity vary depending on the position of the Bragg wavelength.
[0033]
For example, as shown in the upper diagram of FIG. 4, when the reflected light from the FBG sensor is passed through a filter whose transmittance decreases as the wavelength increases, the FBG sensor is subjected to compressive strain (the Bragg wavelength is short). (Shifted to the wavelength side), the transmitted light intensity of the filter increases.
[0034]
Also, as shown in the lower diagram of FIG. 4, when the tensile strain is applied (the Bragg wavelength is shifted to the longer wavelength side), the transmitted light intensity of the filter is lowered. The Bragg wavelength change can be evaluated as the electric signal intensity by converting the transmitted light intensity change into an electric signal by the photoelectric converter.
[0035]
As for the reflected light intensity, the signal intensity changes with the Bragg wavelength change according to the same principle from the relationship of reflectance = 1−transmittance. That is, the transmitted light intensity and the reflected light intensity of the filter change due to strain. However, the intensity received by the photoelectric converter changes each time the optical fiber connector is connected. This is caused by misalignment of the connector connecting portion. For this reason, distortion cannot be quantitatively evaluated by transmitted light or reflected light intensity alone. Strain can be quantitatively evaluated from the value obtained by dividing the difference between the transmitted light intensity and the reflected light intensity by the sum of both intensities.
[0036]
The upper diagram in FIG. 5 is a diagram for explaining the principle of AE detection by the FBG sensor. Since the strain change due to AE is minute, in order to detect AE by the FBG sensor, a filter with a narrow wavelength band in which the transmittance is changed compared to strain measurement is required. The Bragg wavelength of the reflected light from the FBG sensor is slightly changed by AE.
[0037]
This Bragg wavelength change is converted into an intensity change by passing it through a filter having a narrow-band transmittance change. For example, as shown in FIG. 5, when there is no AE, reflected light of the Bragg wavelength λ s is returned from the FBG sensor, and the center wavelength of the transmittance change of the AE detection filter is λ F. The Bragg wavelength changes when the FBG sensor undergoes strain change due to AE. Bragg wavelength changes compressed, and respectively the tensile strain lambda s', and lambda s' on '.
[0038]
The transmitted light intensity of the filter changes in proportion to the area represented by diagonal lines due to the strain change due to AE. Therefore, the output of the photoelectric converter that converts the transmitted light intensity of the filter into an electric signal due to the strain change due to AE is as shown in the lower diagram of FIG. Regarding the reflected light intensity, the reflected light intensity changes when AE is detected in the same manner from the relationship of reflectance = 1−transmittance. Also, the difference between the transmitted light and reflected light signal intensity varies with AE on the same principle.
[0039]
As a filter for detecting AE, there are a dielectric multilayer filter and FBG. In the example of this figure, the band-pass filter is used as the AE detection filter, but a low-pass or high-pass filter may be used.
[0040]
FIG. 6 is a diagram illustrating the shift of the tunable filter operating wavelength band in accordance with the strain change. The filter for detecting AE has a wavelength region in which the transmittance is about 0.4 nm.
[0041]
For this reason, when the Bragg wavelength shifts by several nm due to a large strain change, the wavelength range of the reflected light of the FBG sensor deviates from the wavelength range where the transmittance of the filter for AE detection changes. For this reason, in order to perform AE measurement, it is necessary to change the operating wavelength range of the filter for AE measurement according to the strain change measured by the filter for strain measurement.
[0042]
The tunable filter can change the operating wavelength range by an external control signal. By using a tunable filter as the filter for AE measurement, the operating wavelength range of the filter for AE measurement is controlled according to the strain received by the FBG sensor. At this time, distortion information evaluated from a value obtained by dividing the difference between the transmitted light intensity and the reflected light intensity of the strain measurement filter by the sum of both intensities is used for controlling the operating wavelength region of the tunable filter.
[0043]
(Example 2)
FIG. 7 shows a second embodiment of the present invention, which is configured to enable simultaneous multipoint strain and AE measurement by a plurality of FBG sensors. We show a device that measures strain and AE simultaneously at multiple points by arranging FBG sensors with different Bragg wavelengths in series. The reflected light from the FBG sensor array is separated into signals from the respective FBG sensors by an optical demultiplexer and output. In FIG. 7, the optical circulator before the filter and the extraction of the reflected light from the filter are not shown in order to simplify the drawing.
[0044]
FIG. 8 shows a third embodiment of the present invention, and shows a configuration that enables measurement of strain and AE at a specific location by a plurality of FBG sensors. This shows a device that measures strain and AE at the location where a specific FBG sensor is attached by arranging FBG sensors with different Bragg wavelengths in series.
[0045]
Only the reflected light component from the desired FBG sensor is extracted through the tunable filter through the reflected light from the FBG sensor array from the optical circulator. The operating wavelength band of the strain measurement filter is also changed in conjunction with it. In FIG. 8, the optical circulator before the filter and the extraction of the reflected light from the filter are omitted to simplify the drawing.
[0046]
As described above, the embodiments of the strain and AE measuring apparatus using the optical fiber sensor according to the present invention have been described based on the examples. However, the present invention is not limited to such examples, and It goes without saying that there are various embodiments within the scope of the technical matter described.
[0047]
【The invention's effect】
According to the strain and AE measuring device using the optical fiber sensor having the above configuration, both the strain and the AE can be simultaneously measured by one sensor using the FBG sensor.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a principle diagram of FBG.
FIG. 2 is a diagram illustrating the relationship between Bragg wavelength and strain.
FIG. 3 is a diagram illustrating Example 1 of the present invention.
FIG. 4 is a diagram illustrating the operation of the first embodiment.
FIG. 5 is a diagram illustrating the operation of the first embodiment.
FIG. 6 is a diagram for explaining the operation of the first embodiment.
FIG. 7 is a diagram illustrating Example 2 of the present invention.
FIG. 8 is a diagram illustrating Example 3 of the present invention.
Claims (2)
上記ひずみ計測用のフィルタ及びAE検出用のフィルタは、二種類の波長のそれぞれに対応して互いに透過率が異なり、
上記ひずみ計測用のフィルタ及びAE検出用のフィルタの透過光又は反射光は、ブラッグ波長の変化により強度が変化し、これらを光電変換器で電気信号に変換してひずみ変化とAEを同時に検出することを特徴とする光ファイバひずみとAEの計測装置。An FBG sensor comprising an optical fiber in which FBG is written and attached to a subject; a broadband light source for allowing broadband wavelength light to enter the FBG sensor; a coupler for branching reflected light transmitted from the FBG sensor; and the coupler A strain and AE measurement device using an optical fiber sensor comprising a strain measurement filter and an AE detection filter that respectively reflect or transmit reflected light branched by
The strain measurement filter and the AE detection filter have different transmittances corresponding to the two types of wavelengths,
The transmitted light or reflected light of the strain measurement filter and the AE detection filter changes in intensity due to the change of the Bragg wavelength, and these are converted into electrical signals by a photoelectric converter to detect strain change and AE simultaneously. Optical fiber strain and AE measuring device characterized by this.
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